A window into the role of inhibitory and excitatory mechanisms of perception?

The last few years have seen important advances in the better understanding of human cortical physiology. Functional neuroimaging and transcranial magnetic stimulation (TMS) have allowed us to address novel questions and shed different and complementary information on human brain function. In particular, TMS, a non-invasive and virtually painless tool for delivering currents in the brain, has enhanced our understanding of perceptual and motor functions in intact humans. At the same time it has also yielded data on phenomenology, mechanisms, and strategies to modulate these plastic changes in health and disease. Studies using a paired-pulse protocol (Kujirai et al. 1993) allowed detailed investigations of intracortical excitatory and inhibitory interactions in the human motor cortex. This protocol studied the effects of a TMS pulse, delivered at a subthreshold intensity to elicit a motor evoked potential (MEP), on the MEP response to an upcoming suprathreshold TMS pulse. Depending on the interval, the effect of this conditioning pulse may be inhibitory (intracortical inhibition) or excitatory (intracortical facilitation). Abnormalities in these interactions have been described in Parkinson's disease and dystonia (Ridding et al. 1995); differences with control measures have also been described as a function of motor training or learning (Liepert et al. 1998). This protocol has allowed investigators to study mechanisms of both disease and cortical reorganization, for example after amputations (Chen et al. 1998) or motor training processes.